Known in the art is an exhaust pipe for a steam turbine, comprising in the inlet portion thereof a guide or baffle having a tapered inner surface is tapering, its axis coinciding with the longitudinal axis of the turbine, i.e. with the axis of rotation of the rotor. The guide is mounted so that the end of its smaller diameter adjoins the outer ends of the blades of the runner of the last stage of the turbine, while its other end, which has a larger diameter, is disposed at the outlet of the inlet portion of the exhaust pipe (cf. "Steam Turbine Plants of Nuclear Power Stations", ENERGIYA Publishers, Moscow, U.S.S.R., 1978, pp. 176-177).
However, this known exhaust pipe with the tapering guide would not provide for stable performance of the last stage of the turbine when the operating conditions of the latter varies significantly from the nominal one, i.e. from the duty at at which the turbine has been designed to yield the maximum economy.
When the flow rate by volume of the working fluid varies, e.g. where the working fluid is steam, and/or when the angular speed of the rotor rotation varies, the flow entering a exhaust pipe attains the swirling condition, i.e. the greater the increase in the circumferential and radial components of the working fluid velocity, the more different is the operating condition of the turbine from the nominal one. The centrifugal forces shape the surfaces of the stream of the working fluid into a hyperboloid of rotation, which is assisted by the diffuser effect due to the outwardly tapering shape of the guide. Consequently, in the area of the roots of the blades of the runner, adjoining the inlet of the exhaust pipe, the working fluid stream breaks away off the inner wall of the exhaust pipe, and circulation zones develop, i.e. the zones where the working fluid flows along closed surfaces which are asymmetric with respect of the axis of the turbine, these circulation zones tending to extend as far as into the runner of the turbine's last stage under some operating conditions of the latter.
In this case each blade of the runner successively passes segments with different values of the flow rate by volume of the working fluid in the course of its each revolution, whereby each blade is acted upon by a varying bending effort applied by the working fluid flow. This results in an alternating strain of the blades of the runner and, hence, their eventual untimely fatigue-caused breakdown.
Moreover, the abovementioned circulation zones in the exhaust pipes of steam turbines have been found to cause ingress of water particles, i.e. droplets, from the outlet portion of the exhaust pipe into the runner blade area. This, in its turn, results in the downstream edges of the runner blades becoming eroded away, whereby the cross-sectional area of the blades is reduced and its shape distorted, which further accelerates dangerous conditions, to say nothing of the economy of the operation of the turbine last stage being impaired.
It should be also mentioned that the said circulation zones more often than not are disposed asymmetrically with respect of the axis of the turbine, so that they alter the character of the flow of the working fluid around each blade of the runner in the course of each rotor revolution, which causes additional energy losses in the runner because of the so-called non-stationary character of the flow.